Epidermal growth factor is a polypeptide material that has mitogenic activity for a number of kinds of cells, including epithelial and mesenchymal cells. Epidermal growth factor exhibits epidermal growth promoting activity and gastric acid secretion inhibiting activity, and is therefore useful as a medicament. It has been found that epidermal growth factor loses biological activity in the presence of moisture. This is disadvantageous because such loss of activity makes it impractical to store aqueous preparations of epidermal growth factor for extended periods of time. This invention provides a means for reducing the loss of activity of epidermal growth factor in the presence of moisture.